Overcurrent protection system and sensor used therewith

ABSTRACT

An overcurrent sensor adapted to cooperate with a controller to control the power to a load is shown. In the embodiment illustrated, the controller is an electrical switching means to which the overcurrent sensor furnishes a passive signal in the form of circuit resistance to cause the controller to operate to decrease or increase electrical power to the load circuit at desired current values. The overcurrent sensor comprises a PTC (Positive Temperature Coefficient) thermistor or an NTC (Negative Temperature Coefficient) thermistor mounted in heat transfer relation, through a layer of electrical insulation, with a first heater which is electrically connected in series with an electrical load, and a heat sink member mounted in heat transfer relation through a layer of electrical insulation with the series heater to provide means for changing the transient response time of the thermistor to the series heater current. A second heater is optionally mounted in shunt relation to the first heater to provide a means of increasing the steady state current at which the controller operates, above that obtained solely with the first heater while maintaining approximately unchanged the transient response time of the thermistor assembly for corresponding overload currents.

United States Patent Kilner 1 Oct. 10, 1972 [54] OVERCURRENT PROTECTION[57] ABSTRACT SYSTEM AND SENSOR USED An overcurrent sensor adapted tocooperate with a THEREWITH controller to control the power to a load isshown. In

[72] I t ()li H, Kil W i k, R1, the embodiment illustrated, thecontroller is an electrical switching means to which the overcurrentsensor [73] Assgnee: Texas Instruments Incorporated furnishes a passivesignal in the form of circuit re- Dauas sistance to cause the controllerto operate to decrease [22] Filed: Jan. 4, 1971 or increase electricalpower to the load circuit at desired current values. The overcurrentsensor com- [21] Appl. No.:. 103,406 prises a PTC (Positive TemperatureCoefficient) thermistor or an NTC (Negative Temperature Coeffi- [52]U.S. Cl. ..323/20, 317/41, 318/227, Clem) thermistor mounmd in heattransfer relation 313 399 323 24 323 9 33 24 through a layer ofelectrical insulation, with a first [51] int. Cl .005: 1/44, H02p 5/40hcatcr which is electrically cchhcctcd in series with an 58 Field ofSearch ..323/68, 69, 4, 9, 20, 24; clcchical load, and a heat Sinkmcmbcr mcumcd in 33 23 24; 31 /227 399; 317/33 40 41 heat transferrelation through a layer Of electrical insulation with the series heaterto provide means for 56] References Cited changing the transientresponse time of the thermistor to the series heater current. A secondheater is op- UNITED STATES PATENTS tionally mounted in shunt relationto the first heater to provide a means of increasing the steady statecurrent g t at which the controller operates, above that obtained3505632 4/1970 d 'k' gi "338/23 solely with the first heater whilemaintaining approxi- 3568l25 3/1971 vinemam at a] "338/23 matelyunchanged the transient response time of the thermistor assembly forcorresponding overload cur- FOREIGN PATENTS OR APPLICATIONS rents675,730 0/1952 Great Britain ..338/23 Primary Examiner-Gerald GoldbergAtlorney-Har0ld Levine, Edward J. Connors, Jr.,

John A. Haug, James P. McAndrews and Gerald B.

Epstein 9 Claims, 12 Drawing Figures PATENTEDUBI 10 i972 SHEET 1 BF 4 Inventor,

r e n U H r e U Z Z 0 Any.

OVERCURRENT PROTECTION SYSTEM AND SENSOR USED THEREWITII This inventionrelates to an overcurrent sensor comprising a thermistor and heat sourceand to such a sensor used with a controller to control the operation ofan electrical load at selected values of heater current corresponding tothe resistance of the sensor, which is related to heater current, andcoupled physically, electrically or both with the controller. Among theseveral objects of the invention is the provision of a small compactsensor assembly which can be readily installed and removed forreplacement as needed to provide the desired electrical steady stateoperating current, which is commonly called ultimate trip current,appropriate for the load with which it is used. A further object is theprovision of an overcurrent sensor which can be designed to reach itsoperating point for a wide range of response times, called short timetrip, on a desired value of overload current, which is a currentexceeding steady state operating current or ultimate trip current.Another object is the provision of an overcurrent sensor which will havea short trip time at a desired value of overload current. Still anotherobject of the invention is the provision of an overcurrent sensor whichwill minimize nuisance operation due to premature operation of thecontroller on overload currents when the load is cyclic, intermittent orrepetitive. A further object is the provision of an overcurrent sensorin which the steady state operating current, ultimate trip current, canbe increased with no or little change in short time trip time forcorresponding overload currents expressed as a percent of ultimate tripcurrent.

Other objects and features will be in part apparent and in part pointedout hereinafter.

Conventional state of the art overcurrent sensors of the thermal typeincorporating bimetallic or eutectic alloy elements have designlimitations and difficulties in achieving a short time trip of two tothree seconds in the initial heating cycle from a cold start such asfrom 40 C. room ambient temperature on loads of 600 percent of theultimate trip current of the sensor. Typical industry standards forthermal overload (overcurrent) sensors specify a maximum short time tripof 30 seconds on an overload current of 600, percent of ultimate tripcurrent for standard sensors and short time trip of approximatelyseconds for so called quick trip devices on the same overload current.These short times are related to the heating rate of electric motorwindings on stalled rotor for a typical motor design where stalledcurrent is approximately 600 percent of motor nameplate full loadamperes. In contrast, semiconductor power switching devices such asthyristors' and triacs, which may be used to control motor loads, havelimited thermal capacity on overloads, which are currents exceedingtheir maximum continuous operating current rating, and reach damagingtemperatures in shorter time than electric motors on a similar magnitudeof overload. For example, with an overload of 600 percent of maximumcontinuous operatingcurrent rating at an ambient temperature of 40 C.,the thyristor or triac circuit must have its power decreased in 2 to 3seconds compared to the 10 seconds or longer time acceptable to manyelectric motors carrying stalled current at 600 percent of motornameplate full load amperes and which latter time is provided byconventional motor overload (overcurrent) devices. It will be apparentfrom the following that the present invention obviates the disadvantagesof prior art structures and provides the desired fast short time triptimes.

The present invention, while having more general uses, is particularlyuseful for protecting against overcurrent electric motors as well assemiconductor power switching devices.

The invention accordingly comprises the elements and combination ofelements, features of construction, and arrangements of parts which willbe exemplified in the structures hereinafter described and the scope ofwhich will be indicated in the appended claims.

In the accompanying drawings in which several of the various possibleembodiments of the invention are illustrated:

FIG. 1 is an enlarged view of an overcurrent sensor according to thepresent invention broken away to expose various parts.

FIG. 2 is a side elevation of FIG. 1 with parts broken away.

FIG. 3 is an enlarged perspective view of a thermistor with leadsattached.

FIG. 4 shows a curve of log resistance v. temperature for a PTC(positive temperature coefficient) thermistor and curves of heatercurrent v. temperature.

FIG. 5 shows curves of time for thermistor to reach anomaly range v.percent heater ultimate trip current.

FIG. 6 is a schematic wiring diagram of a typical electrical controllerand its electrical load including an overcurrent sensor.

FIG. 7 is an enlarged view of a second embodiment of an overcurrentsensor according to the present invention broken away to expose variousparts.

FIG. 8 is an end view of FIG. 7 with parts broken away.

FIG. 9 is a partial schematic wiring diagram showing a modification ofthe circuit shown in FIG. 6.

FIG. 10 shows a curve of log resistance v. temperature for a typical NTC(negative temperature coefficient of resistance) thermistor and a curveof heater current v. temperature.

FIG. 11 is a partial schematic wiring diagram showing a modification ofFIG. 6 with an NTC thermistor of an overcurrent sensor connected intothe circuit.

FIG. 12 is a schematic wiring diagram of an overcurrent sensor with PTCthermistor in series with an electrical load.

Similar reference characters indicate corresponding parts throughout theseveral views of the drawings.

Dimensions of certain of the parts as shown in the drawings have beenexaggerated or modified for purposes of clarity of illustration.

Referring to the drawings, particularly FIGS. 1-3, there is shown afirst embodiment of the present invention in the form of an overcurrentsensor generally indicated by numeral 2 comprising a cover 3, athermistor 4 of PTC material, which may be in the form of a cylindricalrod and which will be described later, with leads 6 and 7 providingelectrical series connections to the thermistor at points 10 and 12respectively; a first layer 14 of insulation which may be made ofelectrically insulating material with a desired temperature rating, suchas polyimide film, draped about and in contact with thermistor 4 andextending out therefrom to ends 16 and 17; a heater 18 which may be madeof appropriate high electrical resistance material such asnickel-chromium alloy maintained in contact with thermistor 4 throughfirst layer 14 of insulation by spring tension or other appropriatemeans and extending out therefrom to ends 20 and 21; a second layer ofinsulation 22 which may be made of material similar to layer 14previously described in contact with heater 18 and extending outward toends 24 and 2S, and heat sink 26 which may be made of metal such ascopper in contact with the second layer of insulation 22 by springtension or the like and extending outward to ends 28 and 29. In additionto or in place of spring tension provided by heater 18 and heat sink 26,a layer or dip coating of an adhesive such as electrical varnish can beapplied to establish proper heat flow between aforesaid components aswell as to hold aforesaid parts together. The several layers form agenerally U-shape configuration with the bight portion in close heattransfer relation with thermistor 4 and supported by outwardly extendingends 20 and 21 of heater 18.

A support 34, which may be made of conventional electrically insulativemolded phenolic material, provides a means of attachment for the sensorcomponents and may also provide for the locating and fastening of thesensor in its installed location. Weld pads 30 and 31, comprising aconductive material such as brass, are attached to support 34 byconventional means such as rivets 35 and additionally are electricallyconnected to heater ends 20 and 21, such as by welding. An appropriatecover 3 is shown generally by the broken outline. Terminals (not shown)for connection to lead ends 6 and 7 of thermistor 4 can be installed atany convenient location including support 34 or other appropriate meansfor external circuit connection such as pressure connectors can be used.

Thermistor 4 may be made of positive temperature coefficient (PTC)material such as a lanthanum doped barium titanate for which a typicalcurve 38 of log resistance -v. thermistor temperature is shown inFIG..4. Characteristics of this material is an anomaly where its ohmicresistance increases by several orders of magnitude over a small changein temperature and which, for the material illustrated, occurs atapproximately 1l5-120 C. Different anomaly temperatures may be obtainedby use of different amounts or kinds of dopants in making the thermistormaterials.

Current flow through heater 18 will raise heater temperature and therebytemperature of the thermistor. Relationship of continuous heater currentand steady state heater temperature in a test ambient of 40 C. is shownin FIG. 4 by temperature curve 40 for a particular heater. Steady statetemperature of thermistor 4 corresponds to the temperature of heater 18except for a small gradient. Thus, approximately 2 units of current forheater temperature curve 40 will raise the temperature of thermistor 4into its anomaly point of 120 C. resulting in a thermistor resistance ofohms. This is shown by dotted lines x-x and y-y in FIG. 4 and thecurrent is identified as ultimate trip current Y. If a current exceedingultimate trip current Y is passed through heater 18, the thermistortemperature will reach the anomaly point of 120 C. in some finite timewhich is also known as short time trip and this time will decrease asthe heater current is increased. By assigning a value of percent toultimate trip current Y, the curve 42 in FIG. 5 of percent heaterultimate trip current Y v. time from 40 C. for the thermistor to reachthe anomaly point of C. is obtained. At 600 percent ultimate tripcurrent Y for current 42, short time trip for thermistor to reachanomaly range 120 C. is approximately 12 seconds. If an increase inultimate trip current is desired greater than Y, the ohmic resistance ofheater 18 would be decreased such as by substituting a heater materialof increased thickness. Curve 44 FIG. 4 shows the relationship ofcontinuous heater current and steady state heater temperature at anambient temperature of 40 C., for a heater of increased thicknessmaterial, which raises the thermistor to its anomaly range 120 C. atultimate trip current U. Curve 46, FIG. 5, shows percent heater ultimatetrip current U v. time for the thermistor to reach anomaly point of 120C. At 600 percent ultimate trip current U for curve 46, short time tripfor thermistor to reach anomaly range 120 C. is typically longer thanthe time from curve 42 at 600 percent ultimate trip current Y and thisresults from an increase in the mass of the corresponding heater.Similarly, for a desired decrease in ultimate trip current below Y, theohmic resistance of heater 18 would be increased such as by substitutinga heater material of decreased thickness. Curve 48, FIG. 4, shows therelationship of continuous heater current and steady state heatertemperature at an ambient temperature of 40 C. for a heater of decreasedthickness material which raises the thermistor to its anomaly point of120 C. on ultimate trip current V. Curve 50, FIG. 5, shows percentheater ultimate trip current V v. short time trip for the thermistor toreach the anomaly point of 120 C. At 600 percent ultimate trip current Vfor curve 50, time for the thermistor to reach the anomaly point of 120C. is typically shorter than the time for curve 42 at 600 percentultimate trip current Y. As seen, the overcurrent sensor, with a PTCthermistor as described so far, will reach its anomaly range at adesired ultimate trip current and a change in ultimate trip current willresult in a change in corresponding short time trip.

To reduce the short time trip for thermistor 4 to reach its anomalytemperature on 600 percent ultimate trip current appreciably below the12 seconds of curve 42, a heater of reduced cross section could be usedby using a material of lower resistivity. Another way of reducingtransient response time is to provide a nonuniform cross-section to theheater with a reduced section contiguous with the thermistor and anincreased section elsewhere. Both of these heater modifications resultin an increase in the rate of temperature rise of the heater at thethermistor and, therefore, a decrease in short time trip for thethermistor to reach its anomaly range which is shown by curve 52, FIG.5. However, because of the existence of a temperature gradient, eventhrough it is small, between heater 18 and thermistor 4, the increasedrate of temperature rise in the heater for curve 52 results in theheater attaining a temperature value exceeding the temperature of thethermistor at overload currents such as 600 percent ultimate trip. Thisovershoot in heater temperature beyond the thermistor temperature isobjectionable because it can result in the first insulation 14 reachinga damaging level or in the thermistor reaching its anomaly rangetemperature prematurely. To attain a short time trip as short as 2 to 3seconds at 600 percent ultimate trip current, it is desirable tominimize this temperature overshoot characteristic. By means of theinstant invention this is accomplished by the addition of the previouslydescribed heat sink 26 which is in good heat transfer relationship withheater l8 contacting it through second insulation 22. The effect of heatsink 26 is to quickly reduce the temperature of heater 18 when thecurrent in the heater is reduced after thermistor 4 reaches its anomalyrange, so that the heater temperature will be below the maximumallowable limits for first insulation 14, for thermistor 4, and forsecond insulation 22, to prevent these components from undergoingexcessive thermal degradation and damage.

Overcurrent sensor 2 would usually be electrically connected into anelectrical controller circuit. A typical controller circuit providing anelectrical control function is shown generally by numeral 60, FIG. 6,wherein an increase or decrease in power supplied to the load isprovided by bidirectional thyristor (commonly referred to as triac) 62gated by thyristor 64. The electrical load which can be a motor 66 isshown supplied from the same power source 67 and 68 as electricalcontroller 60. Overcurrent sensor 2 is connected in the controllercircuit so that heater 18 is in series with motor load 66 by means ofterminal connections screws 36 and 37. PTC thermistor 4 is connectedinto the electrical controller circuit as shown in FIG. 6 by means ofleads 6 and 7. The remaining controller circuit components are pushbutton 70, resistors 72, 74, 76, 78, 80, 82, capacitors 84, 86, 88,diodes 90, 92 and zener diode 94. One set of typical values for thecircuit components are as follows:

Items 90, 92, 94, 62, 64 are rated as required. Controller operation isdescribed starting with all components and thermistor 4 at a roomambient temperature such as 40 C. and with motor 66 unloaded. At thistemperature, thermistor 4 is at a low level of resistance such as 500ohms. n energizing power source 67, 68, a voltage is imposed acrosscircuit points 96, 99 which is of the proper value for gatingbidirectional thyristor 62 into its low impedance state therebypermitting power to be supplied to the motor. Voltage difference betweencircuit points 97, 98 is below the breakover voltage of diode 92 andzener 94 so that current in the gate circuit of thyristor 64 is belowits switching level and thyristor 64 is therefore in its high impedancestate. With continuous motor current flowing through overcurrent sensorheater 18, heater temperature and thus thermistor temperature willincrease such as shown by curve 40, FIG. 4. An increase in power to themotor results in an increase in current through the heater and therebyan increase in resistance of thermistor 4 according to curve 38 which inturn increases the voltage across capacitor 86. The controller circuitis calibrated by selection of resistances 76, 78 so that when thermistor4 reaches its anomaly point of 120 C. with the resistance of 10 ohms atultimate trip Y current, the voltage across capacitor 86 is sufficientto switch diode 92 and zener 94 into their low impedance states therebyimposing a voltage across circuit points 98, 99 of the appropriate valuefor gating thyristor 64 into its low impedance state. This results inreducing the voltage across circuit points 96, 99 which reduces the gatecurrent of thyristor 62 resulting in thyristor 62 switching into itshigh impedance state which reduces power supplied to motor 66.Controller circuit 60 will remain in this mode as long as it isenergized. Resetting is accomplished by interrupting power source 67, 68or by opening the controller circuit by means of push button switch 70.This allows capacitor 86 to discharge so that diode 92 and zener 94 willswitch to their high impedance states which reduces gate voltage atthyristor 64 so that it switches into its high impedance state. Thispermits voltage to reappear across circuit points 96, 99 of anappropriate value to gate bidirectional thyristor 62 into its lowimpedance state to increase power supplied to motor 66.

Operation at motor loads resulting in current exceeding ultimate tripcurrent such as ultimate trip Y is defined by curve 42, FIG. 5. Forexample, a motor with a stalled rotor current of 600 percent ultimatetrip current would result in controller 60 reducing power to the motorin approximately 12 seconds starting from 40 C. ambient temperature.This is obtained since this is the short time trip for thermistor 4 toreach its anomaly point of 120 C. at which point controller circuit 60is calibrated for switching thyristor 62 into its high impedance state.

Different motor loads are accommodated by means of heaters havingdifferent characteristics than heater 18 to provide the desired ultimatetrip current in overcurrent sensor 2. Examples of two such changes areultimate trip U and V shown by curves 44, 48 and short time trip timesshown by curves 46, respectively.

Bidirectional thyristors such as 62 typically are limited by internalheating. When conducting a power load such as 600 percent of thyristorcontinuous current rating, the maximum allowable conduction time isconsiderably shorter than the maximum time during which typical motorsmay safely remain on stalled rotor without overheating. Because of this,it is common to use thyristors with a continuous current ratingexceeding the motor nameplate rated load current since overcurrentsensors typically used with motors provide tripping times related to themotor temperature limitations. Thyristors, at these current levels,generally need to be limited to a conduction time of approximately 2seconds versus approximately 15 seconds for motors, at 40 C. ambienttemperature. In many motor applications, of which a refrigerantcompressor is one example, the nature of the motor load is such that themotor generally reaches normal operating speed in less than I second;otherwise, an abnormal condition would be expected to exist. These maybe a mechanical binding within the compressor or low voltage at themotor terminals which would prevent the motor from starting or runningproperly. In this case, a short time trip of 2 seconds, as shown incurve 52, FIG. 5, for overcurrent sensor 2 with heat sink 26, FIG. I, toreach the anomaly range to cause controller 60, FIG. 6, to reduce powersupplied to load 66, would protect thyristor 62 as well as motor 66againstexcessive temperature. Thyristors rated in accordance with motornameplate current rating could be safely used which would result in asubstantial cost savings because of a lower rated thyristor resulting inanother advantage of the instant invention.

One of the design problems concomitant with changing heater 18 toincrease or decrease ultimate trip current Y of overcurrent sensor 2 inthe manner previously described is that the short time trip curve suchas 52 will also change. In some cases it may be desirable to maintainthe short time trip curve essentially unchanged. This is provided by asecond embodiment of overcurrent sensor shown generally at numeral 100,FIG. 7. This second embodiment overcurrent sensor 100 comprises similarcomponents to those of sensor 2, namely, cover 3, thermistor 4, firstand second layers of insulation 14, 22, heater 18, heat sink 26, support34, weld pads 30, 31 and terminal screws 36, 37 and an additional shunt102 connected electrically in parallel with heater 18 at weld pads 103,104 such as by welding. The particular physical location of thecomponents and their characteristics results in either a negligible or adesirable amount of heat from shunt 102 to be conducted to heat sink 26and heater 18. Since shunt 102 and heater 18 are essentially pureresistance at the frequencies of the power source 67, 68 with which theywould normally be used, such as d.c. through 400 cycles per second, thedivision of current between the two circuits by Ohms Law is given by:

where Is current in shunt Ih current in heater Rs resistance of shunt Rhresistance of heater IL current between terminals 36, 37 Heater 18 isselected to provide the minimum contemplated ultimate trip current suchas Y from curve 40, FIG. 4, and the required short time trip on overloadcurrent such as curve 52, FIG. 5. Thermistor 4 will reach its anomalypoint of 120 C. whenever continuous current in heater 18 is at ultimatetrip current Y. The corresponding current in the circuit supplying powerto the motor, in which overcurrent sensor 100 is connected, isdetermined by the resistances of shunt 102 and of heater 18. In a designin which negligible heat is transferred from shunt 102 to heater 18,temperature of thermistor 4 is determined essentially by continuouscurrent through heater 18, such as from curve 40, FIG. 4, and isindependent from current in shunt 102, FIG. 7. Short time trip operationis given by curve 52, FIG. 5, directly in terms of percent IL atultimate trip since the division of current between IL, Ih and Is islinear for all values of IL. This relationship is seen from datarecorded on a series of overcurrent sensors 100 as follows:

2 0.275 ohms0.0909 ohms 6.66 amps 2.2 Secs 3 0.275 ohms0.057 ohms 9.57amps 2.2 Secs 4 0.275 ohms0.035 ohms 13.3 amps 2.2 Secs 5 0.275ohms0.029 ohms 16.9 amps 2.2 Secs No Shunt Further, the objective ofapproximately constant short time trip in terms of percent IL (ultimatetrip) for different resistance values of shunt 102 used with a desiredthermistor 4, heater l8, insulation layers 14, 22 and heat sink 26 isalso obtained when the design permits transfer of heat from shunt 102 toheater 18. The invention, therefore, includes in second embodimentovercurrent sensor a modification in which finite heat transfer takesplace from shunt 102 to heater 18. This is demonstrated by anothersimilar group of overcurrent sensor samples 100 where heater 102 is inthe form of a spiral coil surrounding heat sink 26, heater 18 andthermistor 2 and the data obtained is as follows:

Transient Response Time If the temperature of thermistor 4 wereindependent of heat generated in shunt 102, then IL (ultimate trip)would be calculable from the resistance of a new shunt 102 (rsx) andthat of a sensor with shunt resistance (rs), heater resistance rh andknown IL (ultimate trip) from a formula as follows:

IL (ultimate trip) for No. 11 in the above table based on a shuntresistance of 0.158 ohms referred to No. 10 with a shunt resistance of0.527 ohms and IL (ultimate trip) of 2.15 amps has a calculated value of3.86 amps compared to a measured value of 3.0 amps. Since a linearrelationship exists between currents in the line, shunt and heater, itfollows that current in heater 18 is (3.0/3.86) 100 or 78 percent of theexpected value when thermistor 4 is at its anomaly point of C. for IL(ultimate trip). Referring to FIG. 4, curve 40, which has been used inprevious examples, a heater current of 78 percent of ultimate tripcurrent Y would raise the heater temperature to approximately 103 C.Since thermistor 4 is at its anomaly point of 120 C., heater 18 mustalso be 120 C. and it follows that heat transferred from shunt 102 isproviding an indicated temperature rise of 17 C. at ultimate tripcurrent Y. At 480 percent IL (ultimate trip) for which a short time tripof 2.1 seconds is shown in the preceding data for sensor No. 11 thecurrent in the heater is (480) (78) or 375 percent of its expectedvalue. Referring to FIG. 5, curve 52, the expected short time trip isindicated as approximately 2.8 secs. It is apparent that heat istransferred from shunt 102 to thermistor 4 and heater 18 in order toobtain the shorter time of 2.1 seconds recorded in. the test data. Asimilar analysis can be made for the remaining samples No. 12, 13, 14where it will be seen that heat is transferred from shunt 102 to heaterl8 and thermistor 4 at ultimate trip current Y. The desired value ofshort time as a percent of IL (ultimate trip) is maintainedapproximately constant for different shunts 102 where heat istransferred from the shunt to heater 18. This is confirmed by the testdata which shows short time trip times of 2.0 to 2.1 seconds at 480percent IL (ultimate trip) for samples No. 11 through 14 and is inaccordance with an object of the present invention.

Overcurrent sensor 100, FIG. 7, is electrically connected into anelectrical controller circuit similarly to first embodiment overcurrentsensor 2, FIG. 1. Heater l8 and shunt 102 through terminals 36, 37 areconnected in series with motor 66 and along with FTC thermistor 4 areconnected into electrical controller circuit 60, FIG. 6. Operation withelectrical controller 60 is also similar to that previously describedfor overcurrent sensor 2 and hence the description will not be repeated.Inasmuch as overcurrent sensor 100 operates in cooperation withelectrical controllers such as controller 60 another object of theinvention is met.

Overcurrent sensor 100 can be modified by omitting heat sink 26 and thesecond layer of insulation 22 where short time trip requirements areappropriately long so that overheating of insulation layers 14, 22 andof thermistor 4 is not a problem.

First and second embodiment overcurrent sensors 2 and 100 may also bemodified by substituting a negative temperature coefficient ofresistance (NTC) material, such as nickel oxide manganese oxide, for thepositive temperature coefficient of resistance (PTC) material previouslydescribed for thermistor 4 and the resulting overcurrent sensor,otherwise the same as overcurrent sensor 2, FIG. 2, will be calledovercurrent sensor 110 (not shown) and its thermistor (NTC) 112, FIG.11. Curve 1 14, FIG. 10, shows typical characteristics of log resistancev. temperature of thermistor (NTC) 112. Curve 114 indicates thermistor(NTC) 112 resistance of 0.035 ohms at 120 C. which will be referred toas operating point 120 C. and is shown by dotted lines x and y. Therelationship of heater 18 current and steady state thermistor (NTC) 1 12temperature in an ambient temperature of 40 C. is shown by temperaturecurve 116 for a heater selected so that two units of current raise thetemperature of thermistor (NTC) 112 to 120 C. as shown by dotted lines xand y. This current value is identified as ultimate trip Y.

At heater currents exceeding ultimate trip Y, thermistor (NTC) 112 willreach its operating point 120 C. in some finite time and this time,which is similar to previously identified short time trip, will decreaseas the heater current is increased. Operation of overcurrent sensor 110is similar to that previously described for overcurrent sensor 2 towhich reference may be had for details. Desired values for ultimate tripY as well as for short time trip on a percentage of ultimate trip Ycurrent are obtained similarly to that previously described forovercurrent sensor 2.

However, a change is made in controller circuit 60, FIG. 6, with whichovercurrent sensor (NTC) 110 will be used. Referring to FIGS. 6 and 11,thermistor (NTC) 112 is connected in the circuit in place of resistor 76and a resistor 120 which is used to calibrate the operating point of thecircuit replaces thermistor 4 and resistor 78. Otherwise controllercircuit is unchanged and heater 18 is located at the same point incircuit 60 as shown in FIG. 6. Modified controller circuit 60 iscalibrated so that, when thermistor (NTC) 112 of overcurrent sensorreaches a value of resistance corresponding to its operating point C. atultimate trip Y current, thyristor 64 switches into its low impedancestate which reduces the voltage available at the gate of bidirectionalthyristor 62 so that thyristor 62 assumes its high impedance state. Thecircuit is resettable through push button switch 70 when thermistor(NTC) 112 has cooled, increasing its resistance which lowers the voltageacross capacitor 86, to a point at which zener diode 94 assumes its highimpedance state. Operation of modified overcurrent sensor (NTC) 110 witha thermistor 112 of NTC material is similar to that previously describedfor overcurrent sensors 2 and 100 with thermistors of PTC material atultimate trip currents as well as at currents exceeding ultimate tripand therefore the description will not be repeated.

- It is also within the purview of the invention to employ a separate ordifferent power source for energizing motor 66 in series with heater 18than that energizing the circuit of electrical controller 60, FIG. 6.This is shown in modified controller circuit 120, FIG. 9, otherwise thesame as controller circuit 60, FIG. 6, containing push button station126 and an automatic switching device such as magnetic contact 122comprising coil 123 with circuit connections 124, 125 and load switch128. Contactor 122 is connected into controller circuit 120, FIG. 9, sothat coil 123, in series with push button station 126, is in series withbidirectional thyristor 62 in a circuit connected to power source 67,68. Load switch 128, in series with motor 66 and heater 18, is in acircuit connected to a second power source 130, 131 which may bedifferent than power source 67, 68. Operation is similar to thatpreviously described for controller circuit 60, FIG. 6, except that inmodified controller circuit 120, bidirectional thyristor 62, when in itslow impedance state, energizes coil 123 of magnetic contactor 122energizing the motor and heater through load switch 128. Conversely,when bidirectional thyristor 62 is in its low impedance state, loadswitch 128 is open circuited deenergizing the motor and heater. Thus itwill be seen that the overcurrent sensor of the instant invention alsocooperates with an electrical controller even though the power sourcefor the controller and the load is either separated or different.

The previously described characteristic of overcurrent sensor 2,provided by heat sink 26 in limiting temperature overshoot from heater18 on currents exceeding ultimate trip current, results in the sensorbeing particularly adaptable for use with motors operating at highcyclic loads or with frequent starting and stopping such as in joggingor reversing operation. Since heat sink 26 effectively lowers thetemperature of the heater and thermistor by increasing the heat lossfrom the sensor it follows since the thermistor is less likely toovershoot its anomaly point following cutoff of the cyclic orintermittent high motor current to the sensor heater. This avoids socalled nuisance operation obtained with conventional bimetallic andeutectic alloy overcurrent sensors where the heater temperatureincreases on repetitive cycles at a faster rate than required forprotection of the motor or where the temperature override of the heateris high in relation to the bimetal or eutectic alloy so that heattransfer continues after motor current is shut down which results inunnecessary sensor operation.

Overcurrent sensors 2 and 100 may be used without electrical controllersdirectly with small loads such as fractional horsepower electric motors.In this use, a PTC thermistor with a low value of resistanceapproximately one one-hundredth of the value shown in curve 38, FIG. 4,would be used for thermistor 4 and with heater 18 is connected in serieswith the motor load 66 as shown in FIG. 12. When thermistor 4 reachesits anomaly range, the high increase in its resistance of approximatelythree orders of magnitude, reduces the power supplied to the motorcircuit to a low value so that the motor does not run or overheat andthe sensor performs its intended function. When thermistor 4 has cooledand its resistance has returned to its former low level, power suppliedto the motor is increased so that it will again operate.

As many changes could be made on the above constructions withoutdeparture from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings, shall be interpreted as illustrative and not in a limitingsense, and it is also intended that the appended claims shall cover allsuch equivalent variations as come within the true spirit and scope ofthe invention.

It is to be understood that the invention is not limited in itsapplication to the details of construction and arrangement of partsillustrated in the accompanying drawings, since the invention is capableof other embodiments and of being practiced or carried out in variousways. Also, it is to be understood that the phraseology or terminologyemployed herein is for the purpose of description and not of limitation.

What is claimed is:

1. An overcurrent protection system comprising an electrical controllerswitching means to decrease or increase electrical power to a loadcircuit at predetermined external values of electrical resistance incombination with an overcurrent sensor comprising a housing for supportand installation, a cover, an electrical resistance heater, a positivetemperature coefficient of resistance (PTC) thermistor in heat transferrelation with the electrical resistance heater, a thermal heat sinkmember in heat transfer relation with the electrical resistance heaterthrough electrical insulation to decrease the temperature of theresistance heater upon decrease in heat generation in the resistanceheater, means to electrically connect the resistance heater serially tothe electrical load being switched, means to electrically connect thePTC thermistor to the electrical controller so that an increase in thepower being supplied to the load is effective to increase the resistanceof the thermistor to a first predetermined value sufficient to operatethe electrical controller so as to decrease the supply of power to theload and a decrease in the power being supplied to the load is effectiveto decrease the resistance of the thermistor to a second predeterminedvalue at which power to the load circuit is increased in response tocontroller operation thereby establishing a preselected level of poweracross the load.

2. An overcurrent protection system according to claim 1 including anadditional electrical resistance element in heat transfer relation withand electrically insulated from the thermal heat sink member andelectrically connected in shunt relation with the electrical resistanceheater whereby use of different shunt resistances will result indifferent ultimate trip current levels while maintaining relatively thesame short trip time as a percent of ultimate trip current.

3. An overcurrent protection system according to claim 1 furtherincluding an electrical resistance element physically separated from theassembly of the thermal heat sink member, the electrical resistanceheater and the thermistor, the electrical resistance element beingelectrically connected in shunt relation with the electrical resistanceheater.

4. An overcurrent protection system according to claim 1 including meansto manually reset the controller to permit its operation when theresistance of the thermistor has reached its second predetermined value.

5. An overcurrent protection system according to claim 1 including meansto automatically reset the controller to permit its operation when theresistance of the thermistor has reached its second predetermined value.

6. An overcurrent protection system comprising an electrical controllerswitching means to decrease or increase electrical power to a loadcircuit at predetermined external values of electrical resistance incombination with an overcurrent sensor comprising a housing, anelectrical resistance heater, a temperature responsive resistor in heattransfer relation with the electrical resistance heater, a thermal heatsink member in heat transfer relation with the electrical resistanceheater to decrease the temperature of the electrical resistance heaterthereby preventing premature raising of the temperature of the heatresponsive resistor and concomitant nuisance tripping of the controllerdue to the effect of minor transient overcurrents while maintaining ashort trip time upon sustained overloads, means coupling the resistanceheater to the electrical load being switched, means coupling the heatresponsive resistor to the electrical controller so that an increase inthe power being supplied to the load is effective to increase thetemperature of the heat respon sive resistor and cause a change inresistance to a first predetermined value sufficient to operate theelectrical controller so as to decrease the supply of power to the loadand a decrease in the power being supplied to the load is effective todecrease the temperature of the heat responsive resistor and cause achange in resistance to a second predetermined value at which power tothe load circuit is increased in response to controller operationthereby establishing a preselected level of power across the load.

7. An overcurrent protection system according to claim 6 including anadditional electrical resistance element electrically connected in shuntrelation with the electrical resistance heater whereby use of differentshunt resistances will result in different ultimate trip current levelswhile maintaining relatively the same short trip time as a percent ofultimate trip current.

8. An overcurrent protection system according to 9. An overcurrentprotection system according to claim 7 in which the heat responsiveresistor has a posiclaim 7 in which the heat responsive resistor has ative temperature coefficient of resistance (PTC). negative temperaturecoefficient of resistance (NTC).

l l t I

1. An overcurrent protection system comprising an electrical controllerswitching means to decrease or increase electrical power to a loadcircuit at predetermined external values of electrical resistance incombination with an overcurrent sensor comprising a housing for supportand installation, a cover, an electrical resistance heater, a positivetemperature coefficient of resistance (PTC) thermistor in heat transferrelation with the electrical resistance heater, a thermal heat sinkmember in heat transfer relation with the electrical resistance heaterthrough electrical insulation to decrease the temperature of theresistance heater upon decrease in heat generation in the resistanceheater, means to electrically connect the resistance heater serially tothe electrical load being switched, means to electrically connect thePTC thermistor to the electrical controller so that an increase in thepower being supplied to the load is effective to increase the resistanceof the thermistor to a first predetermined value sufficient to operatethe electrical controller so as to decrease the supply of power to theload and a decrease in the power being supplied to the load is effectiveto decrease the resistance of the thermistor to a second predeterminedvalue at which power to the load circuit is increased in response tocontroller operation thereby establishing a preselected level of poweracross the load.
 2. An overcurrent protection system according to claim1 including an additional electrical resistance element in heat transferrelation with and electrically insulated from the thermal heat sinkmember and electrically connected in shunt relation with the electricalresistance heater whereby use of different shunt resistances will resultin different ultimate trip current levels while maintaining relativelythe same short trip time as a percent of ultimate trip current.
 3. Anovercurrent protection system according to claim 1 further including anelectrical resistance element physically separated from the assembly ofthe thermal heat sink member, the electrical resistance heater and thethermistor, the electrical resistance element being electricallyconnected in shunt relation with the electrical resistance heater.
 4. Anovercurrent protection system according to claim 1 including means tomanually reset the controller to permit its operation when theresistance of the thermistor has reached its second predetermined value.5. An overcurrent protection system according to claim 1 including meansto automatically reset the controller to permit its operation when theresistance of the thermistor has reached its second predetermined value.6. An overcurrent protection system comprising an electrical controllerswitching means to decrease or increase electrical power to a loadcircuit at predetermined external values of electrical resistance incombination with an overcurrent sensor comprising a housing, anelectrical resistance heater, a temperature responsive resistor in heattransfer relation with the electrical resistance heater, a thermal heatsink member in heat transfer relation with the electrical resistanceheater to decrease the temperature of the electrical resistance heaterthereby preventing premature raising of the temperature of the heatresponsive resistor and concomitant nuisance tripping of the controllerdue to the effect of minor transient overcurrents while maintaining ashort trip time upon sustained overloads, means coupling the resistanceheater to the electrical load being switched, means coupling the heatresponsive resistor to the electrical controller so that an increase inthe power being supplied to the load is effective to increase thetemperature of the heat responsive resistor and cause a change inresistance to a first predetermined value sufficient to operate theelectrical controller so as to decrease the supply of power to the loadand a decrease in the power being supplied to the load is effective todecrease the temperature of the heat responsive resistor and cause achange in resistance to a second predetermined value at which power tothe load circuit is increased in response to controller operationthereby establishing a preselected level of power across the load.
 7. Anovercurrent protection system according to claim 6 including anadditional electrical resistance element electrically connected in shuntrelation with the electrical resistance heater whereby use of differentshunt resistances will result in different ultimate trip current levelswhile maintaining relatively the same short trip time as a percent ofultimate trip current.
 8. An overcurrent protection system according toclaim 7 in which the heat responsive resistor has a positive temperaturecoefficient of resistance (PTC).
 9. An overcurrent protection systemaccording to claim 7 in which the heat responsive resistor has anegative temperature coefficient of resistance (NTC).